Recently we found compile time out problem in several cases when SpeculativeLoadHardening was enabled. The significant compile time was spent in register coalescing pass, where register coalescer tried to join many other live intervals with some very large live intervals with many valnos.
Specifically, every time JoinVals::mapValues is called, computeAssignment will be called by getNumValNums() times of the target live interval. If the large live interval has N valnos and has N copies associated with it, trying to coalescing those copies will at least cost N^2 complexity.
The patch adds some limit to the effort trying to join those very large live intervals with others. By default, for live interval with > 100 valnos, and when it has been coalesced with other live interval by more than 100 times, we will stop coalescing for the live interval anymore. That put a compile time cap for the N^2 algorithm and effectively solves the compile time problem we saw.